Former Secretary of State Colin Powell - who has said nary a negative word about the president and administration he served - has finally joined a growing roster of Republican dissidents who have strongly criticized Bush administration policy on the Iraq War and related parts of that crusade.

On the Sunday political show Meet the Press, Powell stated bluntly that the American military detention facility at Guantanamo should be closed down immediately - "this afternoon" to be precise. He also said that the infamous prison where suspected enemy combatants and terrorists have been held incommunicado, mostly without legal counsel and in perpetual legal purgatory, undermines the "American brand" and provides fodder for the world's most disgusting thugs who point to US behaviour at Guantanamo as cover for their own misdeeds.

But Powell's principled call to reverse the wholesale rejection by the US of "rule of law" at Guantanamo is not the only thing that has people chattering. Rather, observers are stunned that the sphinx-like Powell, who has until this week kept his powder dry, came out of hibernation - or perhaps self-censorship - and offered a serious critique of Bush administration policy, something he has strenuously avoided since he left government service in January 2005.

This silence and loyalty to Bush has always seemed out of place given the well-publicised ambivalence Bush and Karl Rove felt towards Powell, who they saw as a threatening political force that they needed to neutralise. But Powell's long-time aide and former chief of staff at the state department, Lawrence Wilkerson, helped lay the groundwork for Powell to eventually break with Bush. Wilkerson cleared his throat on the "Cheney-Rumsfeld cabal" and the utter collapse of the national security decision-making process in a globally reported speech before the New America Foundation on October 19, 2005.

Numerous sources have reported that Powell was not pleased with his former aide's public comments, but in many ways Wilkerson's sizzling speech was a vital part of the effort to preserve Powell's legacy and indicate that he didn't sign off on the administration's efforts to institutionalise soft torture techniques or ignore the Geneva Conventions. The speech also indicated that Powell was, at a minimum, misled on the intelligence that fuelled his presentation on Iraq's WMD efforts before the United Nations - a presentation that was a key moment in the preparation for war.

Wilkerson saved Powell's reputation in the eyes of some people who have yearned to know what the general-turned-diplomat really thought of Bush and his policies. Wilkerson became a proxy for Powell's views - a placeholder, until we learned more from Powell himself. And because Wilkerson broke with him to tell the inside story, and euphemistically fell on his sword to save Powell, the former secretary of state was able to remain almost perversely loyal to the president - whether the president wanted him to or not.

That was vintage Powell: he believes in the preservation of a certain decorum around the Office of the President - no matter who that president is or how wrong-headed that president's policies may be.

But why has Powell finally spoken? Is it guilt for his long-term silence about how America's national security portfolio has been critically degraded? Has the common sense of Lawrence Wilkerson's every-now-and-then e-mail exchanges with Powell moved him forward? Is he miffed that George Tenet, John Bolton, and others are sculpting the "public version" of their mutual history before he does?

I don't think that any of these is the case.

Colin Powell is applying the well-known "Powell Doctrine" in this debate over Guantanamo, and perhaps on the larger subject of the character of this administration.

The "Powell Doctrine" counsels extreme caution in global military affairs. The doctrine insists that before military force is applied, a country must think through every last option (particularly diplomatic); fix the objectives for military action very clearly to avoid 'mission creep'; build alliances and international support for the action; plan for the post-war period and document and make clear the lines defining a clear exit strategy; and, if necessary, apply overwhelmingly massive amounts of force to assure an unambiguous victory.

But there is nuance to Powell's views on military action - or, I would suggest, political action, which is the sort of action he took when he declared that Guantanamo should be shuttered.

The Powell Doctrine is really about applying leverage only when it matters - at a decisive point, or tipping point designed to maximize chances of success. The administration had to falter to a point where Powell's comments would matter enough for him to feel justified in spending political capital. If he had spoken earlier it might have produced no effect.

Some won't understand Powell's reticence. "Too little, too late," they will say. But I don't agree. Colin Powell cleaned up a lot of messes for President Bush, particularly during the early part of his first term. As his political leverage diminished, his comments and his own influence were best kept under wraps until they could matter.

Powell knew the time had come to weigh in - judiciously, with nuance. Just a few words could have a tidal impact. And indeed they did: Powell's defection seriously de-legitimises Guantanamo and the military commissions process. His commentary matters, and it's an indicator of the fact that the vast majority of those in the public and those in power are rejecting Cheney and the warped national security impulses he embraced. From illegal wiretaps to cherry-picking intelligence, from torture techniques to extraordinary rendition, Cheney's "war paradigm" is being unplugged - and Powell is ready to help.

One close friend familiar with the inner workings of White House national security decision-making told me that in the first couple of Bush years, the single most important person who helped "fix" problems that America was getting into was Colin Powell. He mattered when he was in the room - and didn't when he wasn't. When he was gone, Rumsfeld and Cheney would undermine the counsel and strategic game plan he had helped the president understand. That's one of the reasons why Powell travelled as little as possible.

In the end, Cheney and Rumsfeld succeeded in eroding Powell's influence as a cabinet member. But by holding back patiently and making his move at the right time, Colin Powell is making a difference that may decisively submerge the legacies of Rumsfeld and Cheney.